1. Field of the Invention
This invention relates generally to systems wherein a liquid flows along a surface of a body. It relates particularly to a process for reducing skin friction, inhibiting the effects of liquid turbulence, and decreasing heat transfer in systems wherein a liquid flows along a surface of a body.
2. Description of Related Art
Skin friction drag accounts for a sizable portion of the hull drag for both surface and fully submerged marine vehicles. Reducing this drag component would have the obvious advantages of increased speed and/or efficiency. One approach to skin friction drag reduction involves using a film or discrete layer of air at the wall to take advantage of the greatly lower density of a near wall gas phase to interfere with the momentum transfer mechanism responsible for skin friction.
While various methods of introducing a near wall air layer in water flow have been attempted, a stable, optimized air layer has never been successfully maintained at speed. Some of the most promising results have been achieved by injecting microbubbles into the turbulent boundary layer. McCormick and Bhattacharyya achieved drag reduction on a body of revolution by creating bubbles on the surface by electrolysis. (McCormick, M. E.; and Bhattacharyya, R.: Drag Reduction of a Submersible Hull by Electrolysis. Naval Engineers Journal, April, 1973.) More recently, Madavan et al, have completed several studies which yielded microbubble friction reductions. See: Madavan, N. K.; Deutsch, S.; and Merkle, C. L.: Reduction of Turbulent Skin Friction by Microbubbles. Phys. Fluids, Vol. 27, No. 11, February, 1984; Madavan, N. K.; Deutsch S.; Merkle, C. L.: The Effects of Porous Material on Microbubble Skin Friction Reduction. AIAA 22nd Aerospace Sciences Meeting, January 1984, Reno, Nev. AIAA Paper No. 84-0348.; and Madavan, N. H.; Deutsch, S.; and Merkle, C. L.: Measurements of Local Skin Friction in a Microbubble-Modified Turbulent Boundary Layer. J. Fluid Mech., Vol. 156, 1985, pp. 237-256.
While these large friction reductions in themselves are impressive, microbubble injection has serious complications which prevent it from being totally viable as a full scale drag reduction method. The main concerns are: (a) buoyancy--the tendency for the bubble sheet to migrate out from the wall several tens of boundary layer thicknesses downstream of the injection point, and (b) the large volumetric air flow requirement to achieve significant friction reduction. Due to dispersion effects such as turbulence, buoyancy and viscous lift, the boundary layer must, for all practical purposes, be filled with microbubbles--a condition which at ship hull depths requires prohibitively large amounts of pumping energy. Accordingly, a thin, low volume sheet of air located at the wall where the velocity gradient is largest and the skin friction is produced would be optimum to yield friction reductions on the order of those achieved by microbubble injection but at a significantly lower air flow rate/power requirement. The production of such is the primary object of the present invention.
Madavan et al in "Reduction of Turbulent Skin Friction by Microbubbles," supra, disclose a procedure of introducing microbubbles into a boundary layer. However, they do not comprehend using grooves and/or selecting surface characteristics in order to retain the air at the water and solid interface. Because of dispersion effects such as turbulence, buoyancy, and viscous lift, the air bubbles float away from the boundary surface. Consequently, to ensure a layer of air bubbles at the surface in effect requires filling the entire depth of the hull of a ship with microbubbles. Such a procedure uses a prohibitive amount of pumping energy.
Bushnell in "Turbulent Drag Reduction for External Flows," AIAA Paper No. 83-0227, examines various methods of reducing drag. Bushnell independently discusses the use of riblets and the use of gas bubbles at the boundary layer to reduce skin friction drag. Bushnell, however, does not combine the two to create a grooved surface which more effectively retains a layer of gas.
Walsh, U.S. Pat. No. 4,706,910 discloses a method of reducing drag which uses micro-geometry longitudinal grooving of the flow surface. Walsh differs from the present invention because Walsh uses the grooves themselves to reduce surface drag, whereas the present invention uses surface grooves as a means of retaining gas at a boundary layer. Walsh makes no mention of using gas in combination with the grooves to reduce skin friction drag.
McCormick, U.S. Pat. No. 3,957,008, discloses a method of using electrolysis to generate hydrogen and other gases which are mixed with water in the boundary layer of a ship along the entire wetted surface of the hull thereof. McCormick differs from the present invention in that it makes no use of a grooved surface to more effectively retain air at a boundary layer. Rather, McCormick relies solely on a series of pairs of wires placed transversely along the centerline of the hull. The wires produce gases which mix with water in the boundary layer along the entire wetted surface of the hull, thereby reducing drag. In contrast, the present invention uses grooves and selects surface characteristics in order to more effectively maintain gas at the boundary layer.